The goal of this project is to provide proof of principle that estradiol-mediated enhancement of cognitive function can be restored (a) in young rats with cholinergic lesions, and (b) in aged rats that have undergone long-term loss of ovarian function, by treating with a cholinesterase inhibitor and thereby enhancing cholinergic activity in the brain. We hypothesize that the critical period for eliciting positive effects of estradiol on cognitive performance post menopause is defined by the functionality of basal forebrain cholinergic projections (i.e., responsiveness is lost when the cholinergic system becomes significantly impaired). Based on this, we predict that enhancing the cholinergic system pharmacologically (e.g., via the use of cholinesterase inhibitors) will re-open the window of opportunity and restore responsiveness, even after prolonged loss of ovarian function. Selective lesions of cholinergic neurons in the medial septum and diagonal band of Broca will be produced in young ovariectomized rats using the selective immunotoxin 192IgG-saporin (SAP) and methods established in our laboratory. These rats will be treated with specific doses of donepezil or galantamine (cholinesterase inhibitors commonly used in the treatment of Alzheimer's disease), with and without estradiol, and then studied using in vivo microdialysis and behavioral training. Aged rats that are ovariectomized at 3 month of age, and then treated at 12 months of age with donepezil or galantamine with and without estradiol, will also be evaluated. All rats will be trained on two cognitive tasks, a delayed matching-to-position (DMP) T-maze task, and a configural association (CA) operant conditioning task. In vivo microdialysis will be used to measure effects on acetylcholine release in the hippocampus. Levels of choline acetyltransferase and acetylcholinesterase activities in the hippocampus and frontal cortex also will be measured as indices of the degree of cholinergic denervation. Our prediction is that in rats with cholinergic lesions, and in aged rats, effects of estradiol will be restored by treatment with the cholinesterase inhibitors, and that these effects will correlate with AChE inhibition and with acetylcholine release in the hippocampus. This would provide proof of principle that enhancing cholinergic activity in the brain can reinstate the ability of estradiol to enhance cognitive performance both in young rats with impaired basal forebrain cholinergic function, and in aged rats that have undergone long-term loss of ovarian function. PUBLIC HEALTH RELEVANCE Both human and animal data suggest that the timing of hormone therapy relative to menopause is critical for determining whether therapy will have a beneficial effect on brain aging and cognition. We hypothesize that the critical period for eliciting positive effects of estradiol on cognitive performance post menopause is defined by the functionality of basal forebrain cholinergic projections (i.e., responsiveness is lost when the cholinergic system becomes significantly impaired). Based on this hypothesis, we predict that enhancing the cholinergic system pharmacologically (e.g., via the use of a cholinesterase inhibitor) will re-open the window of opportunity and restore beneficial effects of hormone therapy on cognitive performance (a) in young rats with cholinergic lesions, and (b) in aged rats that have undergone long-term loss of ovarian function. This pilot project will provide proof of principal for this hypothesis. Positive results would identify a mechanism to explain why the timing of hormone therapy post menopause is critical, and would provide a viable strategy for restoring the effectiveness of hormone therapy in postmenopausal women who have not used hormone therapy for many years.